This invention relates to methods and apparatus for bonding a prevulcanized rubber tread strip to the circumference of a tire casing. In particular it relates to improved methods and apparatus for obtaining a uniform bond over the entire contact area between the tread strip and the tire casing and for preventing distortion or unwanted movement of a portion of the tread strip and/or bonding medium during the bonding operation.
In recent years it has become conventional to retread a worn tire casing by bonding to the circumference thereof a prevulcanized profiled tread strip by means of rubber-base bonding materials which can be vulcanized rather rapidly at relatively low temperature. The bonding technique is also suitable for making new tires from newly manufactured tire casings having no tread on their circumferences. In either case the technique includes the step of pressing or forcing the tire casing and tread strip together while applying vulcanizing heat to the bonding medium, and in order to obtain a satisfactory bond it is always necessary to avoid the presence of air between the tread strip and tire casing. Removal of air is most readily effected by covering the tread strip and at least the adjacent side walls of the tire casing with a flexible impervious cover or envelope, sealing the cover to the tire and effecting a differential pressure between the interior and exterior of the cover as by applying fluid pressure to the exterior of the cover or applying a vacuum to the space between the cover and the assembly of tire casing and tread strip or a combination of both fluid pressure and vacuum applied sequentially or simultaneously. In either case the air is exhausted through a suitable conduit which is in communication at one end with the space. The pressing and vulcanizing operation may be carried out using the same fluid pressure differential technique or by means of mechanical pressure. Examples of different air expulsion and/or pressing operations are more fully described in U.S. Pat. Nos. 2,976,910 (Nowak), 3,236,709 (Carver), 3,325,326 (Schelkmann), 3,752,726 (Barefoot), and 3,745,084 (Schelkmann).
It has been recognized, for example, in the aforenoted U.S. pat. Nos. 3,325,326 and 3,745,084 that the pressing together of the tread strip and tire casing may sometimes produce variations in the pressure being applied at different locations, with the result that the bonding medium and/or the tread are distorted or shifted during bonding. The resulting product may be unsatisfactory in having a nonuniform bond between tread and tire casing or in having other defects. The problem is thought to arise because pressure is exerted by whatever pressing element is used only on the outer surfaces of the tread profiles. In spite of the air removal operation some air may remain in the tread profiles with a resulting reduction in the pressure transmitted to those portions of the tread strip and those portions of bonding material lying directly below the circumferential tread grooves. The solution proposed by U.S. Pat. 3,325,326 is to first apply a vacuum inside the envelope and fluid pressure to the exterior of the envelope and then increase the pressure inside the envelope. The solution proposed by U.S. Pat. No. 3,745,084 is to assure that the envelope penetrates to the bottom of the grooves.